A THR prosthesis typically comprises a femoral stem component and a femoral head component. The stem commonly includes a conical element, extending from a neck of the stem, for engagement in a complementary recess within the femoral head.
The femoral head of a THR prosthesis is generally more expensive to manufacture than the stem and often come in a range of sizes to suit different patients. For example, heads are commonly provided with maximum diameters of 38 mm through to 58 mm, in increments of 2 mm. In addition, surgeons usually desire to have a range of head lengths available (e.g. −8, −4, 0, +4, +8 and +12 mm) for each head diameter. In this instance a head length of 0 mm denotes that which would result in the patient's leg length being the same as it was before the operation. Consequently, it is often necessary for hospitals to stock many different sizes of head when only one of these will be required for any given procedure.
In order to address this problem, prosthetic sleeves are often provided to fit over the conical element of the stein so that only one head of each diameter need be provided. The sleeves can be provided in different lengths (e.g. to provide head lengths of −8, −4, 0, +4, +8 and +12 mm) but without the expense of providing so many bespoke heads.
The sleeves are usually configured for a tight frictional fit onto the conical element and a similar frictional fit within the head recess.
Where ceramic components are used, the conical element is often adapted to minimize the risk of a brittle ceramic head from cracking or breaking when the conical element is inserted therein. Accordingly, the conical element may be provided with a series of circumferential grooves and ridges to reduce bursting stresses on the head. However, the grooves and ridges typically cause cavities to appear, in which fluid can nest, leading to crevice corrosion which can weaken the frictional fit between the components.
Today ridges and grooves are commonly machined into mating taper surfaces in hip prosthesis, even in metal only taper junctions, because they can help to even out stresses resulting from an imperfect matching of the mating surfaces. As above, the ridges and grooves are circular and are provided in planes that are orthogonal to the longitudinal axis associated with the taper. However, the two complementary sets of circular disposed ridges and grooves on the taper junction further reduce the frictional fit at the component interface.
It is therefore an aim of the present invention to provide a prosthetic component that ameliorates the afore-mentioned problems.